Conditional tenses are used to speculate about what could happen, what might have happened, and what we wish would happen. In English, most sentences using the conditional contain the word if. Many conditional forms in English are used in sentences that include verbs in one of the past tenses. This usage is referred to as "the unreal past" because we use a past tense but we are not actually referring to something that happened in the past. There are five main ways of constructing conditional sentences in English. In all cases, these sentences are made up of an if clause and a main clause. In many negative conditional sentences, there is an equivalent sentence construction using "unless" instead of "if".
THE ZERO CONDITIONAL
The zero conditional is used for when the time being referred to is now or always and the situation is real and possible. The zero conditional is often used to refer to general truths. The tense in both parts of the sentence is the simple present. In zero conditional sentences, the word "if" can usually be replaced by the word "when" without changing the meaning.
If clause Main clause
If + simple present simple present
If this thing happens that thing happens.
If you heat ice it melts.
If it rains the grass gets wet.
TYPE 1 CONDITIONAL
The type 1 conditional is used to refer to the present or future where the situation is real. The type 1 conditional refers to a possible condition and its probable result. In these sentences the if clause is in the simple present, and the main clause is in the simple future.
If clause Main clause
If + simple present simple future
If this thing happens that thing will happen
If you don't hurry you will miss the train.
If it rains today you will get wet
TYPE 2 CONDITIONAL
The type 2 conditional is used to refer to a time that is now or any time, and a situation that is unreal. These sentences are not based on fact. The type 2 conditional is used to refer to a hypothetical condition and its probable result. In type 2 conditional sentences, the if clause uses the simple past, and the main clause uses the present conditional.
If clause Main clause
If + simple past present conditional or present continuous conditional
If this thing happened that thing would happen
If you went to bed earlier you would not be so tired.
If it rained you would get wet.
If I spoke Italian I would be working in Italy.
TYPE 3 CONDITIONAL
The type 3 conditional is used to refer to a time that is in the past, and a situation that is contrary to reality. The facts they are based on are the opposite of what is expressed. The type 3 conditional is used to refer to an unreal past condition and its probable past result. In type 3 conditional sentences, the if clause uses the past perfect, and the main clause uses the perfect conditional.
If clause Main clause
If + past perfect perfect conditional or perfect continuous conditional
If this thing had happened that thing would have happened.
If you had studied harder you would have passed the exam.
If it had rained you would have gotten wet.
If I had accepted that promotion I would have been working in Milan.
MIXED TYPE CONDITIONAL
The mixed type conditional is used to refer to a time that is in the past, and a situation that is ongoing into the present. The facts they are based on are the opposite of what is expressed. The mixed type conditional is used to refer to an unreal past condition and its probable result in the present. In mixed type conditional sentences, the if clause uses the past perfect, and the main clause uses the present conditional.
If clause Main clause
If + past perfect or simple past present conditional or perfect conditional
If this thing had happened that thing would happen. (but this thing didn't happen so that thing isn't happening)
If I had worked harder at school I would have a better job now.
If we had looked at the map we wouldn't be lost.
If you weren't afraid of spiders you would have picked it up and put it outside.
Source:
https://www.ef.com/wwen/english-resources/english-grammar/conditional/
